Consumers can interact with merchants to conduct various financial payment transactions. For example, a consumer can conduct a transaction with a merchant at a point-of-sale system using cash, a transaction card, or other transaction instrument. Conventional systems can include expensive and bulky financial transaction electronics, as may include a card reader for payment cards (e.g., debit or credit cards), a cash drawer, monitors, keyboards, and various other electronic devices. Some point-of-sale systems require multiple types of card readers and/or complex and bulky card-reading equipment in order to accept multiple types of payment cards. Some systems include a card reader that is inserted into a port, such as the audio jack, of a mobile device to read data off a payment card and provide the information to the mobile device.